The present invention relates to structural panels incorporating honeycomb cores and more particularly to an improved core for such panels.
A wide variety of structural panels incorporating honeycomb cores have been developed, particularly for use as aircraft components. The panels typically include an open-cell honeycomb core having thin surface sheets bonded to opposite sides thereof The assembly results in a low-weight member capable of bearing large compression and shear forces.
The surface or face sheets are commonly made of metals or precured thermoset plastics reinforced with synthetic fibers. Typical sheet thicknesses are in the range 0.010 inch to 0.060 inch. Both face sheets are typically bonded to the core using thermosetting adhesives such as epoxies. The assembled core and face sheets are placed in a heated platen press or mold where the adhesive is allowed to cure under heat and pressure. Some thermosetting plastic face sheets are simultaneously cured and bonded to the core in a single operation without the need for adhesive. The uncured plastic resin of the face sheet liquifies initially when heated and wicks up and around the cell edges of the honeycomb which provides the fillets required to attach the face sheets to the core. The amount of pressure and temperature applied depends mainly on the cure characteristics of the resin and is typically in the range of 20 pounds per square inch (psi) to 200 psi and from 200 degrees Fahrenheit (F) to 600 degrees F.
Metal and thermoset plastic skins are easily damaged by impact and it is therefore desirable to use materials with much better impact resistance. Such materials are available in the thermoplastic resins; however, it is difficult to adhere the cores to the surfaces of these materials--especially the most chemical and heat resistant types such as polyphenylenesulfide (PPS) and polyetheretherketone (PEEK). Consequently, these materials have not gained acceptance for structural honeycomb panels.
The honeycomb cores are fabricated of metal, plastic, and/or paper. Further, the core can be of the "foil/film/sheet", "cast", "extruded", or "heat-formed" type.
Honeycomb fabrication of the "foil/film/sheet" type begins with the stacking of flat or corrugated sheets of web material on which parallel, evenly spaced, adhesive lines called "node lines" are rolled or printed on one or both faces. Cores made from these webs can be further divided into the "expandable" and "corrugated" types.
"Expandable" cores are typically made from flat sheets having node lines on only one side of each web. The sheets are placed on top of each other in such a way that the node lines of each consecutive layer will be positioned between two node lines on the layer below. The completed stack is heated and compressed until the node adhesive has cured and joined all layers. The resultant block is then sliced into smaller sections and expanded by pulling the outer-most sheets in directions generally perpendicular thereto. The sheets expand away from one another at areas between the node lines, and a hexagonal honeycomb structure is created.
The materials used in the expansion method include a wide variety of metallic foils, plastic films, paper sheets, and woven and non-woven fabrics of plastic, carbon, and glass fibers. Some cores, such as those made from fiberglass fabric and other fibrous materials, are subsequently dipped into water or solvent solutions of plastic resins in order to increase or enhance structural properties. However, the materials used in the fabrication of an expandable core must be capable of being adhesively bonded to one another. Further, relatively soft and thin-gauge materials must be used in order to facilitate expansion of the cured block, especially with limited adhesive strengths. The required expansion forces must not exceed the strength of the node adhesive or the individual layers of core material. As a further consideration, suitable materials tend to return to their relaxed state and must therefore be heated to their softening point and cooled while expanded so that the materials retain the desired hexagonal open-cell shape. The node adhesive must retain sufficient adhesive and cohesive properties to counteract the expansion forced at the required heat setting. This requirement limits the use of the expandable core to low temperature plastics with surfaces that permit bonding.
Expanded cores fabricated of metal are made mostly of thin aluminum foils of less than 0.006 inch thickness. The resultant cores are rather fragile and need to be handled very carefully prior to bonding. Damage to the unbonded cell walls causes the core to become prefailed, meaning that the core in the damaged area will not reach maximum strength levels. The thin foils are also very sensitive to corrosive environments, such as saltwater. Protective coatings are expensive and often of limited value.
Honeycomb cores made from thermosetting resins perform much better when exposed to corrosion. However, they are not impervious to vapor transmission and thus allow the entry of vapor into the honeycomb through the cell walls. Condensation of vapors is believed to cause the corrosion of adjoining aluminum face sheets within the laminated panels.
The "corrugated" process of honeycomb manufacture is normally used to produce products in the higher density range and to permit the use of materials that cannot be expanded as described above. In the corrugation process, a flat sheet or web is corrugated so that each sheet resembles half of the hexagon shape. Adhesive is then applied to the raised portions of the corrugated sheet; and the sheets are placed on top of each other so that all raised and coated corrugations come into contact with each other creating hexagonally shaped cells. The stacked block is then compressed and heated until the adhesive is cured to join the individual corrugated layers. The corrugated process is also used when cell shapes other than hexagons are desired, for example, to create bell-shaped cells.
The materials used in the corrugated process are typically of greater gauge and bending resistance than the materials used in the expansion process. Typical materials include stainless steel and materials with impregnating resins and binders such as fiberglass and paper. Alternatives to the adhesive joining of the corrugated sheets include spot-welding and solder-dipping, which are typically used with the stainless steel.
The corrugated process is not readily susceptible to automation since the light and often flexible materials used are difficult to support and align during the stacking process. The corrugated sheets tend to nest instead of resting on opposed raised portions. Further, only limited pressure can be used to compress the adhesive films during curing or the cell pattern of the stacked sheets will be distorted. This limited pressure also requires blocks made according to this method to be typically limited in dimension.
"Cast" honeycomb core is fabricated by either (1) pouring a solvent solution of a plastic into a mold resembling a honeycomb pattern or (2) injecting a melted or liquid resin into a mold. Both methods require the material to either dry, cool, or cure inside the mold which typically causes a certain amount of shrinkage. The solid honeycomb core is therefore difficult to unmold. Mold-release provisions are therefore necessary, such as release tapers along the surfaces parallel to the honeycomb cell wall. The resultant cores have tapered cell walls and are restricted to cell diameters sufficiently large to accommodate release tapers. Present typical "cast" cell diameters are at least approximately 0.5 inch.
"Extruded" honeycomb core is produced by forcing a melted plastic through an extrusion die orifice which resembles either a single honeycomb cell or multiple honeycomb cells. The extrusions are cooled, cut to the desired length, stacked, and either adhesively bonded to each other or fused by solvent cementing. The process requires plastic materials that can be either dissolved by solvents or adhered using adhesives with little bonding pressure and/or heat. The extruded materials cannot be easily modified to include fiber reinforcements into the melt stream exiting the extrusion die. The resultant cores are therefore not as strong as those created using the expanded and corrugated techniques.
The "heat-formed" honeycomb cores resemble the typical hexagonal honeycomb core structure the least. According to this method, a thermoformable plastic sheet is heated to its forming temperature and then formed into a honeycomb-like shape by simultaneously stretching the sheet in opposite directions perpendicular to its initial plane. The stretching tools are either chilled metal pins extending through the sheet or perforated platens which are adhered to the plastic sheet and pull it apart. The heat-formed method results in cores with cell walls which are not perpendicular to the cell opening and which have varying cell-wall thicknesses. These cores therefore provide only low structural properties.